Add test scenario for repo discovery with http proxy

[jameerpathan111]

This PR adds test scenario for discovering yum and docker type repo with Http proxy set in Satellite settings.

Test result:

pytest tests/foreman/api/test_http_proxy.py
============================= test session starts ==============================
collected 3 items

tests/foreman/api/test_http_proxy.py ...                                 [100%]
================== 3 passed, 76 warnings in 218.11s (0:03:38) ==================

Note:

• Depends on https://github.com/SatelliteQE/nailgun/pull/839
• I'll be raising separate pr for ui, which will test repo creation and sync.